A process for preparing a tetraester of 3,3',4,4'-biphenyltetracarboxylic acid which comprises a step of dimerizing an o-phthalic diester in the presence of a catalyst comprising a powdery palladium salt and a basic bidentate ligand compound and one or more steps of continuing the dimerization after supplemental addition of at least one of the powdery palladium salt and the basic bidentate ligand compound is already known.
The above-mentioned process is advantageous in that it gives an increased ratio of dimerization (or conversion) of o-phthalic diester and an increased selectivity to the desired tetraester of 3,3',4,4'-biphenyltetracarboxylic acid.
For instance, according to a working example of Japanese Patent Publication No. 5-73733, the ratio of dimerization (conversion) of o-phthalic diester is 23%, and the selectivity to tetraester of 3,3',4,4'-biphenyltetracarboxylic acid is 79%. It is noted, however, that a catalyst yield (i.e., yield based on the amount of Pd employed) is approximately as much as approximately 100 molar times. This catalyst yield is not satisfactory. Moreover, in the process, not a small amount of metallic palladium is deposited on a wall of a reaction vessel. The deposition of the metallic palladium adversely effects the yield of the desired product and disturbs continuing a reaction for a long period of time.